1. Field of the Invention
This invention relates to a thermal printer and a recording method thereof, and more particularly relates to a thermal printer and a recording method thereof provided with an encoder that detects a linear scale marker when a carriage having a thermal head is moved to generate pulse signals intermittently and provided with a control unit that generates and supplies a head pulse corresponding to the pulse signal generated by the encoder to the heater elements of the thermal head.
2. Description of the Related Art
In the technical field of the thermal printer provided with a carriage having a thermal head on which a plurality of heater elements are arranged that controls heating of the thermal head to thereby print a record composed of an aggregate of dots on a recording paper, various modifications have been introduced so that recording is performed on a printing paper by the thermal head with a constant dot pitch regardless of fluctuation in the moving speed of the carriage.
In detail, the thermal printer is provided with the carriage on which a thermal head is mounted having a plurality of heater elements, and the carriage is supported slidably so as to be reciprocated along a platen. Near the platen, a long linear scale comprising markers and blanks having the same length are formed alternately and continuously along the platen. The liner scale is provided in parallel to the platen.
At the position that is facing the linear scale of the carriage, an encoder for detecting the marker of the linear scale when the carriage is moved is provided. The encoder is provided with a light emitting element for emitting a light onto the linear scale and a light receiving element for detecting the light reflected on the marker.
The marker is reflective in nature, and the encoder detects the marker by detecting the reflected light from the marker when the encoder moves to the position of the marker. The encoder generates the detection result of the marker as a pulse signal.
On the other hand, the blank is transparent in nature, and the encoder does not detect the blank. The encoder does not generate a pulse signal when the encoder moves to the position of the blank.
The thermal printer is provided with a memory for storing the encoder signal that is the pulse signal generated by the encoder.
Furthermore, the thermal printer is provided with a control unit for generating and supplying the head pulse that is synchronous with the pulse signal to the heater elements of the thermal head.
Otherwise, the thermal printer is provided with a control unit for generating and supplying the head pulse composed of ON head pulse that is synchronous with the output starting timing of the pulse signal and OFF head pulse that is synchronous with the output ending timing of the pulse signal to the heater elements of the thermal head.
In the case that the conventional thermal printer having the structure described herein above is used recording, when the thermal printer is fabricated or before recording, at first the carriage is moved from the left end to the right end of the platen and the encoder detects the marker of the linear scale. At that time, the encoder generates the pulse signal intermittently every time when the maker (refer to the upper diagram in FIG. 12) is detected. Pulse signals generated intermittently by the encoder are components of a series of encoder pulse signal as shown in the middle diagram in FIG. 12.
The length of each marker of the linear scale shown in the upper diagram of FIG. 12 is formed of a constant interval and is read in a short time if the moving speed of the carriage is fast. On the other hand, the length of the marker is read in a long time if the moving speed of the carriage is slow. As the result, the length of the marker is represented in the form of the difference in the length of the output pulse signal.
The pulse signal generated by the encoder is stored in the memory.
Next, when the thermal printer is used for recording, the control unit reads out the pulse signal data from the encoder.
As shown in the lower diagram of FIG. 12, the head pulse that is synchronized with the pulse signal read out by the control unit is supplied to the heater elements of the thermal head.
As the result, even though the moving speed of the carriage fluctuates as shown with a broken line in FIG. 13 from a constant speed v (referred to as theoretical speed hereinafter) shown with a solid line in FIG. 13, the output timing of the head pulse is synchronized with the output timing of the pulse signal of the encoder.
As described herein above, even though the moving speed of the carriage fluctuates, the dot scattering or overlapping will not occur from the theoretical view point because each dot pitch P recorded by the thermal head can be equalized to the length of one marker of the linear scale. Using the method described herein above, a recorded image of good quality with reduced recording density non-uniformity (so-called jitter) caused in the recording direction is obtained.
However, in the conventional thermal printer, the applied voltage V and the current flow time W of the head pulse are the same for all head pulses. Furthermore, the moving speed of the carriage and the recorded density are in the negatively proportional relation as shown in FIG. 14, and the moving speed of the carriage and the recorded area are in the positively proportional relation as shown in FIG. 15.
As the result, as shown in FIG. 16, the recorded area of one dot recorded at the recording position where the moving speed of the carriage is faster than the theoretical speed is larger than the recorded area of one dot (referred to as theoretical recorded area hereinafter) recorded at the recording position where the moving speed of the carriage is equal to the theoretical speed, and the recorded density of one dot recorded at the recording position where the moving speed of the carriage is faster than the theoretical speed is lower than the recorded density of one dot (referred to as theoretical recorded density hereinafter) recorded at the recording position where the moving speed of the carriage is equal to the theoretical speed. On the other hand, the recorded area of one dot recorded at the recording position where the moving speed of the carriage is slower than the theoretical speed is smaller than the theoretical recorded area, and the recorded density of one dot is higher than the theoretical recorded density.
The recorded area and the recorded density can not be equalized at all the recording positions and jitter can not be removed perfectly, and the recorded image of good quality can not be obtained disadvantageously.
An another conventional thermal printer, in which the head pulse that is synchronized with the output starting time and the output ending time of the pulse signal read out by the control unit is supplied to the heater elements of the thermal head with motion of the carriage along the platen, has been known.
In this case, for the purpose of convenience, the time point shown with A in the upper diagram of FIG. 17 is assumed to be the output starting time of the pulse signal and the time point shown with B in the upper diagram of FIG. 17 is assumed to be the output ending time of the pulse signal. The length of each marker of the linear scale shown in the middle diagram of FIG. 17 is the relative length viewed from the encoder. In detail, because the encoder reads one marker in a shorter time at the position where the speed of the carriage is fast, the length of the marker is shorter. On the other hand, because the encoder reads one marker in a longer time at the position where the speed of the carriage is slow, the length of the marker is longer.
In the conventional thermal printer, due to various causes such as the accuracy of the linear scale, the accuracy of a light emitting diode light source of the encoder, and the positional accuracy of the encoder and the linear scale, the discrepancy between the output time TON and the non-output time TOFF of the pulse signal as shown in the upper diagram of FIG. 18 is a problem even when the moving speed of the carriage is constant, in other words, when the resolution of the output is to be doubled by using both starting and ending signals, unlike the case in which only the starting signal of the encoder is used, the signal accuracy becomes poor.
To solve the problem, a current is supplied to the heater elements by means of the head pulse based on both starting and ending of the encoder signal. However, the print dot pitch P is not constant as shown in the lower diagram of FIG. 18 and jitter is caused, and the problem is not solved.
The ratio of the output time TON and the non-output time TOFF of one pulse of the pulse signal under an assumption that the moving speed of the carriage is constant is called generally as duty ratio. It has been known from the statistical data of all encoder signals that the TON and TOFF tend to exhibit normal distributions having respective average values different from each other. In detail, as shown in FIG. 19, TON and TOFF tend to converge to consistent average values such as 40% and 60% respectively based on the total of TON and TOFF of 100%. Accordingly, the duty ratio TON/TOFF converges to a consistent value for all pulse signals.
It is an object of the present invention to provide a thermal printer and a recording method of the thermal printer that are capable of obtaining the recorded image of good quality with no jitter by means of a method in which the head pulse is synchronized with the pulse signal generated by an encoder and at least one of the applied voltage and the current flow time of the head pulse is controlled.
In detail, the thermal printer of the present invention is provided with a control unit having an arithmetic unit for operating at least one of the applied voltage and the current flow time of the head pulse based on the difference between the theoretical value and the measured value of the pulse signal, and controls the head pulse based on at least one of the applied voltage and the current flow time operated by the arithmetic unit.
Another object of this invention is to control the thermal printer so that the applied voltage of the head pulse is increased and/or the current flow time of the head pulse is shortened at the recording position where the moving speed of the carriage is faster, and on the other hand, the applied voltage of the head pulse is decreased and/or the current flow time of the head pulse is extended.
Furthermore, the thermal printer of the present invention is characterized in that the control unit controls at least one of the applied voltage and the current flow time of the (N+1)-th head pulse based on the error of the N-th pulse signal.
Another object of this invention is to provide a method in which the encoder detects the marker and the control unit generates the head pulse simultaneously.
Furthermore, the recording method of the thermal printer of the present invention is characterized in that at least one of the applied voltage and the current flow time of the head pulse is operated based on the difference between the theoretical value and the measured value of the period of the pulse signal and the head pulse is controlled based on at least one of the applied voltage and the current flow time operated as described herein above.
Another object of this invention is to control the thermal printer so that the applied voltage of the head pulse is increased and/or the current flow time of the head pulse is shortened at the recording position where the moving speed of the carriage is faster, and on the other hand, the applied voltage of the head pulse is decreased and/or the current flow time of the head pulse is extended at the recording position where the moving speed of the carriage is slower.
Furthermore, the recording method of the thermal printer of the present invention is characterized in that at least one of the applied voltage and the current flow time of the (N+1) -th head pulse is controlled based on the error of the N-th pulse signal.
Another object of this invention is to provide a method in which the encoder detects the marker and the head pulse is generated simultaneously.
Another object of this invention is to provide the thermal printer and the recording method of the thermal printer that are capable of obtaining a good quality image with reduced jitter by performing correction in which the output time of the head pulse to be supplied to the heater elements is delayed by a predetermined time from the output starting time or the output ending time of the pulse signal generated from the encoder to correct the time error caused in the encoder.
In detail, the thermal printer of the present invention is characterized in that the thermal printer is provided with a measurement unit for measuring the duty ratio of the output time to the non-output time of the pulse signal generated by the encoder and an output correction time arithmetic unit for operating the output correction time of the OFF head pulse based on the duty ratio measured by the measurement unit, and the control unit generates and supplies the OFF head pulse at the time point that is delayed by the output correction time from the output ending time of the pulse signal to the heater elements.
Another object of this invention is to provide a good quality image having a constant dot pitch by correcting the output timing of the head pulse to compensate the time error caused in the encoder even if the resolution of the encoder is doubled.
The thermal printer of the present invention is characterized in that the thermal printer is provided with a measurement unit for measuring the duty ratio of the output time to the non-output time of the pulse signal generated by the encoder and the output correction time arithmetic unit for operating the output correction time of the ON head pulse based on the duty ratio measured by the measurement unit, and the ON head pulse is generated and supplied at the timing that is delayed by the output correction time from the output starting time of the pulse signal to the heater elements.
It is the object of the present invention to provide the image of good quality having a constant dot pitch by correcting the output timing of the head pulse to compensate the time error caused in the encoder even if the resolution of the encoder is doubled.
The recording method of the thermal printer of the present invention is characterized in that the duty ratio of the output time to the non-output time of the pulse signal generated by the encoder is measured, the output correction time of the OFF head pulse is operated based on the measured duty ratio, and the OFF head pulse is supplied to the heater elements at the time point that is delayed by the output correction time from the output ending time of the pulse signal.
It is the object of this invention to provide the image having a constant dot pitch by correcting the output timing of the head pulse to compensate the time error caused in the encoder even if the resolution of the encoder is doubled.
The recording method of the thermal printer of the present invention is characterized in that the duty ratio of the output time and the non-output time of the pulse signal generated by the encoder is measured, the output correction time of the ON head pulse is operated based on the measured duty ratio, and the ON head pulse is supplied to the heater elements at the time point that is delayed by the output correction time from the output starting time of the pulse signal.